Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

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2 Module 5: Lecture -5 on Stability of Slopes

3 Slope stabilization methods

4 Slope stabilization methods generally reduce driving forces, increase resisting forces, or both. Driving forces can be reduced by excavation of material from the approximate part of the unstable ground and drainage of water to reduce hydrostatic pressure acting on the unstable zone. Resisting forces can be increased by: (1) Drainage that increases the shear strength of the ground (2) Elimination of weak strata or the potential failure zone (3) Building of retaining structures or other supports (4) Chemical treatment to increase shear strength of the ground.

5 Unloading Unloading is a type of slope stabilization technique to reduce the driving forces within a slide mass. Excavation is a common method for increasing stability of a slope by reducing the driving forces that contribute to movements: This can include: (1) Removing weight from the upper part of the slope (2) Removing all unstable or potentially unstable materials (3) Flattening slopes (4) Benching

6 Flattening slopes Flattening the slope not only reduces driving forces, but tends to force the failure surface deeper. 1 2 L1 L2

7 Rock-fill buttresses A simple method to increase slope stability is to increase the weight of the material at the toe, which creates a counterforce that resists failure. Riprap instead of soil is preferable, however, because it has a greater frictional resistance to shear forces and is also free draining

8 EPS-Block Geofoam in Slope Stabilization Failure Modes External instability Internal instability Pavement system failure Major components of an EPS-block geofoam slope system (After Arellano et al. 2009)

9 Static and Seismic Slope Stability (Existing Soil Slope Material Only) Static and Seismic Slope Stability (Both Fill Mass and Existing Soil Slope Material) (After Arellano et al. 2009)

10 External Seismic Stability Failure Involving Horizontal Sliding of the Entire Embankment (After Arellano et al. 2009) External Seismic Stability Failure Involving Overturning of an Entire Vertical Embankment about the Toe of the Embankment

11 Internal Seismic Stability Failure Horizontal Sliding Load Bearing Failure of the Blocks (After Arellano et al. 2009)

12 Drainage Techniques Adequate drainage of water is the most important element of a slope stabilization scheme, for both existing and potential slopes prone to failure. Drainage is effective because it increases the stability of the soil and reduces the weight of the sliding mass. Drainage can be either surface or subsurface. Surface drainage can be through either surface ditches or shallow subsurface drains. Surface drainage is especially important at the head of the slide, where a system of cutoff ditches that cross the headwall of the slide, and lateral drains to lead runoff around the edge of the slide are effective.

13 Drainage Techniques The FOS against on any potential slip surface that passes below the phreatic surface can be improved by subsurface drainage. Methods that can be used to accomplish subsurface drainage are: a) Drainage blankets b) Trenches c) Cut-off drains d) Horizontal drains e) Relief wells to lower the water pressures in layers that are deep down in the subsoil (can t be reached by open excavation) a) Drainage tunnels or galleries when there is a requirement of substantial number of horizontal drains

14 Slope stabilization using retaining walls Soil-tyre retaining wall Gabion retaining wall

15 Slope stabilization using retaining walls

16 Slope stabilization using retaining walls After Grundbau und Bodenmechanik (2001)

17 Slope stabilization using vertical piles A row of stabilizing piles embedded within a slope that is prone to failure

18 Slope stabilization using vertical piles Discrete pile row used to stabilise a potentially unstable slope After Yoon and Ellis (2009)

19 Slope stabilization using vertical piles After Ashour and Ardalan (2012) Driving force induced by displaced soil mass above the sliding surface

20 Slope stabilization using anchors The permanent grouted anchors have been extensively used to provide vertical and lateral support for natural and engineered structures during the past 6 decades. The end type of anchorage, where the tendon is grouted below the potential slip surface, has been used to stabilize dangerous slopes to a specified safety factor because of its significant technical advantages resulting in substantial cost savings and reduced construction period.

21 Slope stabilization using anchors Safety factor of slopes with anchors After Cai et al. 2003)

22 Slope stabilization using anchors The safety factor of slopes stabilized with anchors can be calculated by the following two approaches: (1) a vertical effect approach conventionally used in practice, and (2) a normal effect. Thus the safety factor for the vertical effect approach is given by where P is the axial tension per unit width, and θ is an angle to indicate the orientation of Anchors.

23 Slope stabilization using anchors Safety factor for the normal effect approach can be obtained by dissolving axial tension in the anchor into two components, namely normal and tangential to the base of the slice, where the slip surface intersects the anchor. The tangential component of the axial tension was assumed to have no influence on the normal force at the base of the slice where the slip surface intersects the anchor;

24 Slope stabilization using anchors The reinforcing mechanism of anchors in slopes can be explained using the additional shearing resistance, induced by the axial tension, on the slip surface. The additional shearing resistance was given more rationally by the normal approach than the conventional vertical approach The stabilizing effect was optimal in a range of the angle θ from 7.5 to 22.5 and at the anchor position 2m horizontally from the crest for 1V:1H slope. (Cai and Ugai, 2003)

25 Slope stabilization using anchors After Vaciago (2012)

26 Slope stabilization using stone columns Stone columns to stabilize an unstable slope (After Abramson et al. 2002)

27 Slope stabilization using stone columns

28 Slope stabilization using stone columns Stability calculations are carried-out by using conventional slope stability analysis methods. The basic equations for developing average shear strength values are:

29 Slope stabilization using stone columns

30 Slope Stabilization for Berths at Gangavaram Port, India After Keller Ground Engineering India Pvt Ltd (2007)

31 Slope stabilization using injected lime slurry (After National Lime Association, 1985)